With the advancement of communication technologies, the applications using communication technologies have also increased significantly, thus making the related products more diversified. Especially, consumers have more demands on advanced functions from communication applications, so that many communication applications with different designs and functions have been continuously appearing in the market, wherein the computer network products with wireless communication functions are the main streams recently. Moreover, with integrated circuit (IC) technologies getting matured, the size of product has been gradually developed toward smallness, thinness, shortness and lightness.
An antenna in the communication products is an element mainly used for radiating or receiving signals, and the antennas used in the current wireless products have to own the features of small size, excellent performance and low cost, so as to be broadly accepted and confirmed by the market. Generally, the features of antenna can be known by the parameters of operation frequency, radiation pattern, return loss, and antenna gain, etc. With regard to a common wireless apparatus, such as an access point (AP), a dipole antenna is generally used therein, and is formed on a printed circuit board (PCB) for data communication. Since the common dipole antenna has a longer size, it occupies more space on the printed circuit board, and also the cost for fabricating such an antenna is relatively higher. The monopole antenna is one of the simplest antennas in structure, and thus is suitable for use in a wireless apparatus requiring small size. For example, in a USB wireless card, the size of the antenna has to be small for fitting.
Reduction of the monopole antenna's physical size reduces the operating radiation bandwidth of the antenna. In order to overcome this problem caused from size reduction, several conventional monopole antennas having such as helical radiating elements and a sleeve surrounding the monopole radiating element, etc. have been provided. However, it has been difficult to manufacture these conventional antennas within strict tolerance requirements. Moreover, even though these conventional antennas may reduce the antenna's physical length, they still have the adverse effect of inherently increasing the diameters thereof, thus actually increasing the antenna's overall size.
Moreover, it is quite difficult for the conventional monopole antennas to simultaneously have the feature of low cost, small size, high antenna gain, broad operation bandwidth and good radiation patterns, so that the applications of the conventional monopole antennas are greatly limited.
Hence, there is an urgent need to develop a monopole antenna for satisfactorily meeting the antenna requirements of low cost, small size, high antenna gain, broad operation bandwidth and good radiation patterns, thereby overcoming the disadvantages of the conventional monopole antenna.